The present invention relates to a method for producing cerium oxide, a cerium oxide abrasive, a method for polishing a substrate using the same, and a method for manufacturing a semiconductor device, and more specifically relates to a method for producing cerium oxide particles with high productivity and yield, a cerium oxide abrasive that can provide high speed polishing without causing scratches irrespective of the film properties, a method for polishing a substrate using the same, and a method for manufacturing a semiconductor device having high reliability with high productivity and yield.
In a manufacturing process of a semiconductor device, as a chemical-mechanical polishing method for smoothing an inorganic insulating layer such as an SiO2 insulating layer, which is formed by a plasma CVD method, or low-pressure CVD method, a CMP method has been conventionally used. As an abrasive used for the CMP method, a colloidal silica series abrasive or slurry using silica particles, cerium oxide particles or the like as the abrasive grain is used.
The colloidal silica series abrasive is produced by grain-growing silica particles by a process of pyrolyzing silicic acid tetrachloride or the like and pH adjusting the silica particles with an alkaline solution containing no alkali metal such as ammonia, etc. However, such an abrasive has a technical problem that the polishing speed on an inorganic insulating film is not sufficient and higher polishing speed is required for practical use.
On the other hand, as a glass-surface abrasive for a photomask, a cerium oxide abrasive has been used. Cerium oxide particles have lower hardness than silica particles or alumina particles, therefore it tends to cause few scratches on the surface to be polished and thereby it is useful in finishing mirror polishing. Further, as the cerium,oxide has been known as a strong oxidant, it has chemically active properties. Thus, by utilizing these advantages of the cerium oxide, the application of it to a chemical-mechanical abrasive for the insulating film is useful.
However, when a cerium oxide abrasive is applied to inorganic insulating film polishing as it is to polish a glass surface for a photomask, a problem arises that it causes scratches to a degree that can be visually noticed on the insulating film surface as the diameter of the primary grain is large. Further, there is another problem that some types of a cerium oxide abrasive greatly change its polishing speed depending on the film properties of a surface to be polished.
An object of the present invention is to provide a method of easily producing cerium oxide that can polish a surface to be polished such as an SiO2 insulating film, etc., at high speed and good yield without causing scratches.
Another object of the present invention is to provide a cerium oxide abrasive having cerium oxide as an essential component that can polish a surface to be polished such as an SiO2 insulating film, etc., at high speed irrespective of the film properties without causing scratches.
Yet another object of the present invention is to provide a method for polishing a substrate that can polish a surface to be polished such as an insulating film, etc., at high speed irrespective of film properties without causing scratches.
Still further object of the present invention is to provide a method for manufacturing a semiconductor device in which a semiconductor device having excellent reliability can be manufactured with high productivity and yield.
The present invention relates to a method for producing cerium oxide comprising rapid heating of cerium salts to a calcining temperature to calcine them.
Further, the present invention relates to the method for producing cerium oxide in which the temperature rise rate of raising to the calcining temperature is set to 20 to 200xc2x0 C./min.
Further, the present invention relates to the method for producing cerium oxide in which the calcining is performed by a rotary kiln.
Further, the present invention relates to the method for producing cerium oxide in which the calcining temperature is set to 600 to 1,000xc2x0 C. and the calcining time is set to 30 minutes to 2 hours.
Further, the present invention relates to a cerium oxide abrasive containing the cerium oxide produced by the above-mentioned method for producing cerium oxide and pure water.
The present invention also relates to an abrasive containing a slurry in which cerium oxide particles having an intensity ratio of the area of a primary peak appearing at 27 to 30xc2x0 to that of a secondary peak appearing at 32 to 35xc2x0 (primary peak/secondary peak) of 3.20 or more in a powder X-ray diffraction chart are dispersed into a medium.
Further, the present invention relates to an abrasive containing slurry in which abrasive grains having pores are dispersed into a medium.
Further, the present invention relates to an abrasive containing slurry in which cerium oxide particles having a bulk density of 6.5 g/cm3 or less are dispersed into a medium.
Further, the present invention relates to a method for polishing a substrate comprising polishing a predetermined substrate using the above-mentioned abrasive.
Further, the present invention relates to a method for manufacturing a semiconductor device comprising the step of polishing a semiconductor chip on which a silica film is formed with the above-mentioned abrasive.
The cerium salts to be used in the present invention may include cerium carbonate, cerium sulfate, cerium oxalate and the like. These cerium salts may be hydrates. From the viewpoint of producing a cerium oxide easily with good yield, which is an essential component of a cerium oxide abrasive that can polish a surface to be polished such as an SiO2 insulating film, etc., at high speed without causing scratches, cerium carbonate is preferably used and cerium carbonate hydrate is more preferably used as the cerium salts.
It is preferred that these cerium salts be in the form of powder in terms of workability.
In the method for producing cerium oxide of the present invention, it is necessary to rapidly heat cerium salts to raise the temperature to a calcining temperature and calcine them.
When gradual heating of the cerium salts is conducted to raise the temperature to a calcining temperature and they are calcined, the obtained cerium oxide does not have desired performance, and a cerium oxide abrasive using such cerium oxide is likely to cause scratches on the surface to be polished and high speed polishing becomes difficult.
Here, the temperature rise rate of raising the temperature of cerium salts to a calcining temperature is preferably set to 20 to 200xc2x0 C./min, and more preferably to 40 to 200xc2x0 C./min.
Further, while the calcining process may be performed in a batch furnace, it may be preferably performed by a rotary kiln.
Further, the calcining temperature is preferably set to 600 to 1,000xc2x0 C.
Further, the calcining time is preferably set to 30 minutes to 2 hours.
The rotary kiln is an already known furnace, and a type of the rotary kiln is not limited. There may be mentioned, for example, a material in which a refractory lined cylindrical kiln is provided such that the axis of the kiln is inclined relative to the horizontal line, both ends of the kiln are rotatably supported by the upper and lower side supporting members respectively, and the rotary kiln is rotatably driven through a driving apparatus such as a motor and a gear attached to the output shaft of the driving apparatus with a ring-shaped gear mounted on the periphery of the kiln.
In case of using a rotary kiln, a preferred embodiment is as follows. The temperature in the rotary kiln duct is defined as 600 to 1,000xc2x0 C. and the rotary kiln is previously heated to the temperature. Then cerium carbonate hydrate is charged into the kiln duct at a predetermined weight per hour, and the rotary kiln is rapidly heated at a temperature rise rate of 20 to 200xc2x0 C./min. At that time, the filling rate of cerium carbonate relative to the cross-sectional area of the kiln duct is defined as 3 to 10%. Further, a predetermined flow rate of oxygen gas or the like is blown into the kiln duct and the heating of the hydrate is carried out in an oxidizing atmosphere.
It is preferred that an abrasive according to the present invention contains a slurry in which cerium oxide particles having an intensity ratio of the area of a primary peak appearing at 27 to 30xc2x0 to that of a secondary peak appearing at 32 to 35xc2x0 (primary peak/secondary peak) of 3.20 or more in a powder X-ray diffraction chart are dispersed into a medium.
In cerium oxide particles dispersed in the slurry in a cerium oxide abrasive of the present invention, the intensity ratio of the area of a primary peak appearing at 27 to 30xc2x0 to that of a secondary peak appearing at 32 to 35xc2x0 (primary peak/secondary peak) is preferably 3.20 or more, more preferably 3.20 to 4.20, and most preferably 3.30 to 4.00 from the powder X-ray diffraction chart. Here, the diffraction angle or peak intensity of scattered X-ray obtained by the powder X-ray diffraction reflects the properties relating to atoms which constitute the crystal and their arrangement, and identification of a crystalline substance and structural analysis of crystallizability or the like can be made from the diffraction chart.
The cerium oxide according to the present invention exhibits a cubic system and the primary peak appearing at 27 to 30xc2x0 in the powder X-ray diffraction chart is analyzed as a [1,1,1] plane and the secondary peak appearing at 32 to 35xc2x0 therein is analyzed as a [2,0,0] plane.
When a strain is caused by oxygen defect or the like in the crystal of the cerium oxide particles, the strain toward the [1,1,1] plane is increased and the main peak intensity for 27 to 30xc2x0 is decreased. Thus, the intensity ratio of the area of a primary peak appearing at 27 to 30xc2x0 to that of a secondary peak appearing at 32 to 35xc2x0 (primary peak/secondary peak) is decreased. If the intensity ratio of the area of a primary peak appearing at 27 to 30xc2x0 to that of a secondary peak appearing at 32 to 35xc2x0 (primary peak/secondary peak) is less than 3.20, the polishing speed can be rapidly decreased in some cases according to film properties of a surface to be polished.
Here, as a measuring device for the powder X-ray diffraction chart, a commercially available device (for example, Geigerflex, trade name, produced by Rigaku) can be used.
The larger the primary grain diameter of the cerium oxide particle is, and the less the crystalline strain is, i.e., the better the crystallizability is, the higher speed polishing becomes possible with respect to an SiO2 insulating film formed by TEOS-CVD method or the like. However, polishing scratches are likely to occur. Accordingly, the cerium oxide particles in the present invention are preferably prepared without enhancing the crystallizability thereof significantly. Further, since the cerium oxide abrasive is used for polishing semiconductor chips, the content of alkali metals and halogens in cerium oxide is preferably limited to 1 ppm or less.
In the abrasive of the present invention, the content of Na, K, Si, Mg, Ca, Zr, Ti, Ni, Cr, and Fe is preferably each 1 ppm or less respectively, and the content of Al is preferably 10 ppm or less.
The cerium oxide particles according to the present invention can be produced by calcining, for example, a cerium compound. However, calcining at a low temperature that does not increase the crystallizability of the cerium particle as much as possible is preferably used for preparing cerium oxide particles, which do not cause scratches on the surface thereof.
The cerium oxide obtained by calcining can be ground by dry grinding with a jet mill or the like, or by wet grinding with a bead mill or the like. The ground cerium oxide particles include single crystalline particles having a small crystalline size, and ground particles which have not been yet ground to the crystalline size. The ground particles are different from an aggregate obtained by reaggregating the single crystalline particles, and comprise two or more crystallites having grain boundaries. When polishing is performed by an abrasive containing the ground particles having the grain boundaries, the stress on polishing breaks the boundaries and it is assumed that an active surface of crystal is continuously generated. Thus, a surface to be polished such as an SiO2 insulating film can be polished at high speed without causing scratches.
The cerium oxide abrasive according to the present invention contains cerium oxide produced by the above-mentioned method for producing cerium oxide and pure water.
The cerium oxide abrasive according to the present invention can be obtained by mixing the cerium oxide particles produced by the above-mentioned method, pure water and a dispersant used as required thereby to disperse the cerium particles. The cerium oxide particles may be, if necessary, classified with a filter or the like. Here, while the concentration of cerium oxide particles is not restricted, it is preferably in a range of 0.1 to 10% by weight, and more preferably in a range of 0.5 to 10% by weight from the viewpoint of easy handling of a suspension (an abrasive).
Dispersants, which can disperse cerium oxide particles into a medium, may be used without limitation. The dispersants, which do not contain metallic ions, may include, for example, (meth)acrylic acid polymer and its ammonium salts; water-soluble organic polymers such as polyvinyl alcohol, etc.; water-soluble anionic surfactants such as ammonium lauryl sulfate, polyoxyethylene lauryl ether ammonium sulfate, etc.; water-soluble nonionic surfactants such as polyoxyethylene lauryl ether, polyethylene glycol monostearate, etc.; and water-soluble amines such as monoethanolamine, diethanolamine, etc. (Meth)acrylic acid in the present invention means an acrylic acid and a methacrylic acid corresponding thereto, and alkyl (meth)acrylate means an alkyl acrylate and an alkyl methacrylate corresponding thereto.
Further, the acrylic acid polymers and their ammonium salts may include, for example, an acrylic acid polymer and its ammonium salt, a methacrylic acid polymer and its ammonium salt, and a copolymer of ammonium (meth)acrylate and alkyl (methyl, ethyl or propyl) (meth)acrylate.
Specifically, poly(ammonium (meth)acrylate) and a copolymer of ammonium (meth)acrylate and methyl (meth)acrylate are preferred. In case the latter is used, the molar ratio of the ammonium (meth)acrylate to the methyl (meth)acrylate, that is, ammonium (meth)acrylate/methyl (meth)acrylate is preferably 10/90 to 90/10.
Further, the acrylic acid polymer or its ammonium salt preferably has a weight average molecular weight (value obtained by measuring with a GPC and calculated in terms of standard polystyrene) of 1,000 to 20,000 and more preferably 5,000 to 20,000. When the weight average molecular weight exceeds 20,000, the change in grain size distribution with the lapse of time due to reaggregation tends to occur. On the other hand, when the weight average molecular weight is less than 1,000, the effects of dispersibility and anti-sedimentation are sometimes insufficient.
The amount of these dispersants to be added preferably ranges from 0.01 part by weight to 5 parts by weight based on 100 parts by weight of cerium oxide particles from the viewpoint of dispersibility and anti-sedimentation properties of particles in slurry. To enhance the dispersing effect, it is preferred to charge the dispersants simultaneously or substantially simultaneously into a dispersion machine together with the cerium oxide particles during the dispersion process. In a case where less than 0.01 part by weight of the dispersant is used based on 100 parts by weight of cerium oxide particles, the cerium oxide particles tend to sediment, and on the other hand, in a case where more than 5 parts by weight of the dispersant is used, the change in grain size distribution with the lapse of time due to reaggregation tends to occur.
In a method of dispersing these cerium oxide particles into water, a homogenizer, an ultrasonic dispersing machine, a ball mill or the like may be used in addition to a usual stirrer.
To disperse cerium oxide particles of a sub-xcexcm order, it is preferred to use a wet type dispersion machine such as a ball mill, an oscillating ball mill, a planetary ball mill, and a medium stirring mill.
If the alkalinity of slurry is to be enhanced, an alkaline substance containing no metallic ion such as aqueous ammonia may be added during the dispersion process or after the process.
To the abrasive according to the present invention, N,N-diethylethanolamine, N,N-dimethylethanolamine, aminoethylethanolamine, anionic surfactants or the above-mentioned dispersants or the like may be added appropriately according to the manner of usage.
The aspect ratio of a primary particle, i.e., a crystallite, which forms the cerium oxide particles dispersed in the cerium oxide abrasive according to the present invention, is preferably 1 to 2, and a median value of 1.3. The aspect ratio can be measured by observation with a scanning type electron microscope (for example, Model S-900 manufactured by Hitachi, Ltd.).
In this abrasive, the cerium oxide particle preferably comprises 2 or more crystallites, and has grain boundaries.
The median value of diameters of cerium oxide particles having grain boundaries is preferably 60 to 1,500 nm, more preferably 100 to 1,200 rim, and most preferably 300to 1,000 nm.
The median value of the diameters of the crystallites is preferably 5 to 250 nm, and more preferably 5 to 150 nm.
Particles having the median value of particle diameters of cerium oxide with grain boundaries of 300to 1,000 nm, and the median value of crystalline diameters of 10 to 50 nm are preferably used.
Further, particles having the median value of particle diameters of cerium oxide with grain boundaries of 300to 1,000 nm, and the median value of diameters of crystallites of 50 to 200 nm are preferably used.
The maximum diameter of a cerium oxide having grain boundaries is preferably 3,000 nm or less and the maximum diameter of crystallites is preferably 600 nm or less, and more preferably 10 to 600 nm.
In the present invention, the crystalline particle diameter and cerium oxide diameter having crystal particle boundaries can be measured by observation with the above described scanning type electron microscope (for example, Model S-900 manufactured by Hitachi, Ltd.). The diameter of the cerium oxide particle, or a slurry particle, can be measured by a laser diffractometry (using, for example, Master Sizer microplus produced by Malvern Instrument Co. Ltd.; refractive index: 1.9285, light source: Hexe2x80x94Ne laser, absorption 0). Furthermore, the particle diameter of a particle can be obtained from the major axis and the minor axis of the particle. That is, the major axis and the minor axis of the particle are measured and the root of the product of major axis and the minor axis of the particle is defined as a particle diameter. Then, the volume of a sphere obtained by the resultant particle diameter is defined as a particle volume.
The median value is one in the distribution of the volume particle diameter, and means a particle diameter when the volume ratio becomes 50% after totalizing the volumes of particles from the smaller diameter of particle.
In a case where the cerium oxide particle is constituted by 2 or more crystallites and has crystal particle boundaries, it is preferred that cerium oxide particles, each of which has a particle diameter of 1 xcexcm or more, occupy 0.1% by weight or more of the total weight of the cerium oxide particles. Such cerium oxide particles can polish a predetermined substrate while being broken during polishing.
The measurement of the content of cerium oxide particles having a particle diameter of 1 xcexcm or more is performed by measuring the intensity of transmitted light shielded by particles using an in-liquid particle counter. As a measuring device, a commercially available device (for example, model 770 Accu-Sizer (trade name) produced by, Particle Sizing System Inc.) can be used.
The cerium oxide particle constituted by 2 or more crystallites having grain boundaries preferably polishes a predetermined substrate while presenting,a new surface which is not yet in contact with a medium during polishing.
Further, it is preferable that the cerium oxide particle constituted by 2 or more crystallites having grain boundaries has a ratio of the content of cerium oxide particles having a particle diameter of 0.5 xcexcm or more measured by a centrifugal sedimentation process after the polishing of a predetermined substrate to the content of cerium oxide particles having a particle diameter of 0.5 xcexcm or more measured by a centrifugal sedimentation process before the polishing of 0.8 or less.
Further, it is preferred that the cerium oxide particle constituted by 2 or more crystallites having grain boundaries has a ratio of the cerium oxide particle diameter of D99% by volume measured by a laser diffractometry after the polishing of a predetermined substrate to the cerium oxide particle diameter of D99% by volume measured by a laser diffractometry before the polishing of 0.4 or more and 0.9 or less.
Further, it is preferable that the cerium oxide particle constituted by 2 or more crystalline grains having grain boundaries has a ratio of the cerium oxide particle diameter of D90% by volume measured by a laser diffractometry after the polishing of a predetermined substrate to the cerium oxide particle diameter of D90% by volume measured by a laser diffractometry before the polishing of 0.7 or more and 0.95 or less.
Here, the D99% and D90% mean particle diameters when the particle diameters become 99% and 90% respectively, after totalizing the volumes of particles from the smaller diameter of particle, in the distribution of the volume particle diameter.
Incidentally, the centrifugal sedimentation method is to measure the intensity of light transmitted through cerium oxide particles settled by centrifugal force to obtain the content of the cerium oxide particles. As the measuring apparatus, for example, SA-CP4L (trade name) produced by Shimadzu Corp.) may be used.
Further, a state after a predetermined substrate has been polished means the state after a predetermined substrate is set on a holder on which a substrate-mounting adsorption pad for supporting the substrate to be polished, and the holder is placed on a platen to which a piece of porous urethane resin polishing cloth is stuck with the surface to be polished down, further a weight is placed thereon so that the working load reaches 300 g/cm2 and the surface to be polished is polished by rotating the platen for a predetermined period of time at a rotation speed of 30 minxe2x88x921 (30 rpm) while dropping the abrasive on the platen at a dropping rate of 50 ml/min. At that time, the abrasive used for polishing is circulated for reuse. The total amount of the abrasive is 750 ml.
The measurement by a laser diffractometry can be performed with Master Sizer microplus (trade name) manufactured by Malvern Instrument Co., Ltd. (refractive index: 1.9285, light source: Hexe2x80x94Ne laser).
The abrasive according to the present invention contains a slurry in which abrasive grains having pores are dispersed into a medium. Here, as the abrasive grains cerium oxide particles are preferably used.
The pore preferably has a pore ratio of 10 to 30% obtained from the ratio between the density measured using a pycnometer and a theoretical density obtained by the X-ray Rietveld analysis. A pore volume measured by B. J. H. (Barret, Joyner, Halende) method is preferably 0.02 to 0.05 cm3/g.
Further, the abrasive according to the present invention contains a slurry in which cerium oxide particles having a bulk density of 6.5 g/cm3 or less are dispersed into a medium. Here, the density is preferably 5.0 g/cm3 or more and 5.9 g/cm3 or less, and as the medium, pure water is preferably used. This slurry can contain a dispersant, and as a dispersant, at least one selected from a water-soluble inorganic polymer, a water-soluble anionic surfactant, a water-soluble nonionic surfactant and a water-soluble amine is preferred, and a salt of a polyacrylic acid polymer can be preferably used, and an ammonium salt thereof can be more preferably used.
The pH of the abrasive according to the present invention is preferably 7 to 10, and more preferably 8 to 9 in the points of polishing properties, dispersibility of cerium oxide particles, anti-sedimentation properties and the like.
Films to be polished by the abrasives according to the present invention may include, for example, inorganic insulating films, specifically, an SiO2 film formed by the CVD method using SiH4 or tetraethoxysilane (TEOS) as an Si source and oxygen or ozone as an oxygen source.
As a predetermined substrate, a semiconductor substrate in a phase of circuit elements and aluminum wirings formed thereon or a semiconductor substrate in a phase of circuit elements formed thereon and further an SiO2 insulating film layer formed thereon can be used. Also, a substrate including an SiO2 insulating film formed for the purpose of semiconductor isolation (Shallow trench isolation) can be used.
By polishing an SiO2 insulating film layer formed on such a semiconductor substrate with the above-mentioned abrasive, depressions and projections on the surface of the SiO2 insulating film layer are removed thereby to form a smooth surface over the entire surface of the semiconductor substrate.
Here, as a polishing device, a typical polishing device having a platen (on which a motor or the like whose number of revolutions is changeable is mounted) to which a holder, which holds a semiconductor device, and a piece of polishing cloth (a pad) are adhered, can be used.
As the polishing cloth, a general nonwoven fabric, an expanded polyurethane, porous fluorine resins or the like can be used without specific limitation. Further, it is preferred that in the polishing cloth a groove in which slurry is stored be formed.
Although the polishing conditions have no limitation, a low rotational speed of 100 minxe2x88x921 is preferred for the rotational speed of a platen so that a semiconductor does not come off, and the pressure applied to the semiconductor substrate is preferably 105 Pa (1 kg/cm2) or less so that no scratches will be present after polishing.
During polishing, a slurry is continuously supplied to the polishing cloth with a pump or the like. Although the supply amount of the slurry is not limited, it is preferred that the surface of the polishing cloth be always covered with the slurry.
Preferably, the polished semiconductor substrate should be rinsed well in running water and water drops attached onto the semiconductor substrate should be shaken off and dried by using a spin dryer or the like. On an SiO2 insulating film layer smoothed in this manner, an aluminum wiring, which is the second layer, is formed, then an SiO2 insulating layer is formed again between the wires and on the wiring by the above-mentioned method, and subsequently the recessions and projections on the surface of the insulating layer are removed by polishing them with the above-mentioned cerium oxide abrasive, whereby a smooth surface is formed over the entire surface of the semiconductor substrate. By repeating this step with predetermined times, a semiconductor having a desired number of layers can be manufactured.
Predetermined substrates according to the present invention include a substrate on which an SiO2 insulating film is formed, a wiring board on which an SiO2 insulating film is formed, an inorganic insulating film such as glass and silicon nitride, an optical glass such as a photomask, a lens and a prism, an inorganic conducting film such as ITO, an optical integrated circuit, an optical switching element, an optical waveguide formed by glass and crystalline materials, an end surface of optical fiber, optical single crystal for a scintillator or the like, a solid laser single crystal, an LED sapphire substrate for blue laser, a semiconductor single crystal such as SiC, GaP, GaAs, etc., a glass substrate for a magnetic disc, a magnetic head and the like. The cerium oxide abrasive according to the present invention is used for polishing the above-mentioned substrates.